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Date: Mon, 26 Sep 2011 19:07:44 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Greg KH <gregkh@suse.de>
Cc: Tejun Heo <htejun@gmail.com>, Luke Macken <lmacken@redhat.com>,
        stable@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH stable-3.0] ptrace: don't clear GROUP_STOP_SIGMASK on
	double-stop
Message-ID: <20110926170744.GA16944@redhat.com>
References: <20110926170632.GA16936@redhat.com>
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On 09/26, Oleg Nesterov wrote:
>
> 1. Luke Macken triggered WARN_ON(!(group_stop & GROUP_STOP_SIGMASK))
>    in do_signal_stop().
>
>    This is because do_signal_stop() clears GROUP_STOP_SIGMASK part
>    unconditionally but doesn't update it if task_is_stopped().
>
> 2. Looking at this problem I noticed that WARN_ON_ONCE(!ptrace) is
>    not right, a stopped-but-resumed tracee can clone the untraced
>    thread in the SIGNAL_STOP_STOPPED group, the new thread can start
>    another group-stop.
>
>    Remove this warning, we need more fixes to make it true.

Tejun, 3.1 has similar problems. I'll write another email...

Oleg.